1. Field of the Invention
The present invention relates to a rotation transmission mechanism employed in the reel base-driving device of a magnetic recording/reproducing apparatus, such as a video tape recorder (VTR).
2. Description of the Related Art
As is well known, a helical scan VTR is provided with both a front loading mechanism and a tape-loading mechanism. When a video tape cassette containing a video tape is horizontally inserted into the cassette insertion port formed in the front face of the VTR, the front loading mechanism receives the cassette by means of a cassette holder and draws the cassette inside together with the cassette holder, and then lowers the cassette until it comes to the predetermined cassette-loading position. When the cassette is being lowered, its cover is opened, and the tape-pulling members of the tape-loading mechanism are inserted into the cassette and brought into contact with the inner side of the tape. Next, the tape-loading mechanism causes the tape-pulling members to pull the tape out of the cassette placed at the cassette-loading position and guides the tape such that it is in contact with about half of the circumference of the rotating cylinder. After the tape-loading mechanism sets the tape along the tape feed path way, various operation modes, such as recording play, freeze (i.e., still image reproduction), slow play, fast-forward play, fast-rewind play, fast forward, and fast rewind, are selectively established with a mode-establishing mechanism and its associated circuits. If an eject key is operated, the tape-loading mechanism draws the tape back into the cassette, and the front loading mechanism returns the cassette from the cassette-loading position to the cassette insertion port.
In this type of magnetic recording/reproducing apparatus, the reel bases having reel shafts adapted for engagement with the reel shaft-engaging holes of a tape cassette is rotated by a reel base-driving device, so as to drive a tape. The reel base-driving device includes a rotation transmission mechanism which transmits the rotation of a capstan motor to the reel bases. With the reel bases driven or controlled by use of torque desirable for an operating mode, the tape is driven in a stable manner in accordance with the operating mode.
In the rotation transmission mechanism of a conventional reel base-driving device, a pulley constituting a belt transmission is arranged at an intermediate point between the reel bases such that the pulley is swingable around a shaft. A driving gear and a driven gear are coaxially mounted on the pulley, with a friction mechanism interposed. This friction mechanism includes a clutch plate, and two friction couplings located on the respective sides of the clutch plate and having frictional forces different from each other. One of the friction couplings is in contact with the driving gear, while the other one is in contact with the driven gear. The clutch plate is coupled to one end of a coupling member rotatably supported by the driven gear. The other end of the coupling member is coupled to the driven gear by use of a coil spring. The coupling member and the coil spring jointly constitute a one-way clutch. With this construction, when the driving gear is rotated, this rotation is first converted, in accordance with the rotating direction, into one of first and second torques which correspond to the frictional forces of the friction couplings of the friction mechanism, and is then transmitted to the driven gear.
In the conventional reel base-driving device, a reel base-driving gear for driving or controlling the reel bases is in mesh with the driven gear of the rotation transmission mechanism by use of a swingable member. The reel base-driving gear is rotated and urged by the swingable member in accordance with the rotating direction and engages with one of the take-up and supply reel bases, to thereby rotate the reel base which it engages with. Thus, the reel base is driven with the first torque in the play and fast-forward play modes of the magnetic recording/reproducing apparatus, and is driven with the second torque in the rewind play and fast-rewind play modes of the apparatus.
In the rotation transmission mechanism, a directly-coupling gear is arranged in opposition to the driving and driven gears such that it can be engaged with the driving and driven gears or disengaged therefrom. This directly-coupling gear is designed to transmit the rotating force of the driving gear directly to the driven gear, to thereby providing the reel base-driving gear with third torque used for quickly driving the tape in the fast-forward and fast-rewind modes of the magnetic recording/reproducing apparatus.
However, this type of rotation transmission mechanism is faced with the following problems. As is shown in FIG. 1A, driving gear 1 and driven gear 2 are driven while being urged in directions opposite to each other (i.e., the directions indicated by arrows A and B). As a result, the teeth of directly-coupling gear 3 which is in mesh with the driving and driven gears are twisted, as is shown in FIG. 1B, resulting in noise during gear engagement. In addition, since the torsion between driving and driven gears 1 and 2 acts in such a manner as to urge directly-coupling gear 3 in a direction away from driving and driven gears 1 and 2, as indicated by arrow C in FIG. 1A, a very large force is required for bringing directly-coupling gear 3 into engagement with driving and driven gears 1 and 2.
As mentioned above, the conventional rotation transmission mechanism has problems, in that the torsion acting between the driving and driven gears causes the teeth of the directly-coupling gear to be twisted, resulting in noise during gear engagement, and in that a very large force is required to control the engagement of the directly-coupling gear since the torsion between the driving and driven gear acts in such a manner as to urge the directly-coupling gear in a direction away from the driving and driven gear.